The present invention relates in general to boilers and in particular to a new and useful boiler water trip system which utilizes the integral of a calculated boiler water level signal to generate a time delay for applying a drum level signal to a tripper for tripping the boiler when the drum level signal indicates the presence of water in the drum at too high and too low a level.
Boilers which generate steam from boiling water and utilize steam drums are known. It is known to provide the steam drums with level transmitters or sensors that generate a signal when water in the drum is at too high or too low a level. At this point equipment connected to the boiler become operable to trip the boiler and terminate dangerous boiler activity.
A boiler trip must be generated when the drum water level drops below the lowest permissible water level at which the boiler can be safely operated. A boiler trip should also be generated when high drum water level exceeds the point at which cyclones that are used with the boiler may be flooded and result in carry-over.
Since drum level is subject to transient variations, instantaneous trips on drum level would result in excessive false trips of the boiler which are not acceptable to operating companies. Normally, a boiler trip signal from drum level equipment is generated from the high or low drum level trip point being exceeded for a fixed period of time. Establishing this time period involves compromises. If the time is set to provide protection under a worst case failure, it may be too short to allow the controls or operator to correct for a less severe failure. Yet if it is set longer to allow time for the controls or operator to correct the problem, it may be too long to protect the boiler under all conditions.
For additional information concerning boiler and drum structure as well as controls therefor see Steam, Its Generation and Use, 39th Ed., Babcock and Wilcox Co., 1978.